1. Field of the Invention
The present invention relates to an image forming apparatus and a process cartridge used therein, and more particularly, to an image forming apparatus that uses a charging device to form electrophotographic images and a process cartridge implementing such image forming capabilities.
2. Discussion of the Background
Charging devices are used in electrophotographic imaging systems, such as photocopiers, facsimiles, printers, or the like, where an electrostatic charge is generated to form an electrostatic latent image on a photoconductive surface.
Typically, such electrophotographic charging devices are equipped with a cleaner that cleans the charging surface to ensure reliable charging performance for an extended period of time. There are several types of charging device cleaners, one common type of which includes a mechanical member that operates in contact with the charging device to remove contaminants by scrubbing or wiping.
It is known that holding the cleaning member continuously in contact with the charging device throughout the operation results in degraded and inefficient cleaning performance. This occurs when contaminants collected on the cleaning member transfer back to the charging device, or when the cleaning member is damaged due to the continuous operation.
To avoid such inefficiency, a conventional charging device cleaner is used with a retraction mechanism that retracts the cleaning member from the charging device so that cleaning is performed at set intervals regulated, for example, by the number of motor rotations or by the number of pages printed since a previous cleaning operation.
In addition, some control methods have been proposed to enhance efficiency in such retractable or intermittently operable cleaners. For example, one such method uses a sensor that detects contamination of a charging device in order to perform cleaning according to the actual level of contamination. Another control method involves determining the degree of contamination based on the imaging system's state of operation, such as consumption of developer material and ambient humidity detected, and performing cleaning according to the contamination degree thus determined.
Moreover, in recent years, high-speed color electrophotographic imaging apparatuses (referred to as “tandem printers”) have come into common use. An electrophotographic color image is obtained by superimposing one atop another toner images of different primary colors. The tandem color printer includes multiple imaging units disposed in tandem with one another, one for black and others for non-black colors, each of which individually performs an electrophotographic process to obtain a toner image of a particular color.
Typically, such multiple imaging units in a tandem printer are subjected to different operating conditions. For example, the black imaging unit, involved in the formation of both black-and-white (monochrome) and full-color images, may be used more frequently than the non-black imaging units, which are not employed in the black-and-white image formation. Moreover, the tandem imaging units in the same printer are not necessarily replaced at a same time. Such difference in replacement timing results when the black imaging unit employed more frequently has a shorter service lifetime than the other imaging units, and when a breakdown occurs in only one imaging unit requiring replacement before the end of its intended service lifetime.
The different operating conditions in the tandem imaging units cause different degrees of contamination of the multiple charging devices included in the imaging process, making it difficult to properly clean each individual charging device. Naturally, a failure in proper cleaning leads to unsatisfactory performance of the charging device and corresponding degradation of imaging quality.